(a) Field of the Invention
The present invention relates to a liquid crystal display (LCD) device having an elevated yield and reliability and a method for fabricating a TFT panel included therein.
(b) Description of the Related Art
A conventional LCD described in JP-A-10(1998)-206888 shown in FIGS. 1 and 2 includes a TFT (thin film transistor) panel 60 and a counter panel 61. The counter panel 61 includes a transparent dielectric substrate 12, a transparent counter electrode 22 and a liquid crystal (LC) orientation layer (not shown) disposed on the counter electrode 22.
The TFT panel 60 includes a transparent dielectric substrate 11 on which a common electrode 24, scan lines 13 and a gate electrode 13a connected thereto are disposed. A gate dielectric film 14, a semiconductor layer 15, signal lines 16, a source electrode 16a connected thereto, a drain electrode 16b, and a passivation layer 17 are disposed on the common electrode 24.
A black matrix 18, a color filter 19 made of photosensitive color resin and an overcoat 20 made of photosensitive transparent resin are disposed on the passivation layer 17. Through-holes are formed in the color filter 19 and the overcoat 20.
A transparent pixel electrode 21 connected to the drain electrode 16b through the through-hole is disposed on the overcoat 20.
The TFT panel 60 and the counter panel 61 are disposed so that the respective LC orientation layers not shown are opposed to each other, and an LC layer 23 is interposed between the LC orientation layers.
FIGS. 3A to 3D show consecutive steps for fabrication of the LCD of FIGS. 1 and 2.
The common electrode 24, the scan lines 13 and the gate electrode 13a are formed on the transparent dielectric substrate 11 by means of patterning as shown in FIG. 2. After formation of the gate dielectric electrode 14 on the common electrode 24, the drain electrode 16b and the passivation film 17 are formed.
Then, after the color filter 19 is formed on the passivation film 17 as shown in FIG. 3A and exposed to light by employing a photomask 100 as shown in FIG. 3B, etching is conducted as shown in FIG. 3C to form the overcoat 20 and the transparent pixel electrode 21 as shown in FIG. 3E, thereby forming the TFT panel 60 shown in FIG. 2.
Then the LCD shown in FIG. 2 is fabricated by disposing the LC layer 23 between the counter panel 61 and the TFT panel 60 and bonding the TFT panel 60 and the counter panel 61 with each other.
Another conventional LCD described in JP-A-9(1997)-152625 shown in FIGS. 4 and 5 similarly includes a TFT panel 60 and a counter panel 61.
The counter panel 61 includes a transparent dielectric substrate 12, a black matrix 18, a color filter 19, a transparent counter electrode 22 and an LC orientation layer (not shown) disposed on the counter electrode 22.
The TFT panel 60 includes a transparent dielectric substrate 11 on which a common electrode 24, scan lines 13 and a gate electrode 13a connected thereto are disposed. A gate dielectric film 14, a semiconductor layer 15, a signal line 16, a source electrode 16a connected thereto, a drain electrode 16b, and a passivation layer 17 are disposed on the common electrode 24.
An interlayer dielectric film 25 made of photosensitive transparent resin is formed on the passivation layer 17. A transparent pixel electrode 21 connected to the drain electrode 16b through a through-hole is disposed on the interlayer dielectric film 25.
The TFT panel 60 and the counter panel 61 are disposed so that the respective LC orientation layers not shown are opposed to each other, and an LC layer 23 is interposed between the LC orientation layers.
The LCD is fabricated in accordance with the following procedures.
The interlayer dielectric film 25 overlying the transparent dielectric substrate 11 is formed by means of exposure, development and baking by employing the photosensitive transparent resin having a thickness of about 4.5 .mu.m prepared by its application and pre-baking.
The reason for forming the interlayer dielectric film 25 at the thickness of 4.5 .mu.m is as follows. When a coupling capacitance between the signal line 16 and the pixel electrode 21 is small, the potential of the pixel electrode 21 is affected by the signal line 16. The large thickness of the interlayer dielectric film 25 can suppress the potential variation of the pixel electrode 21.
The amount of the potential variation (.alpha.) is expressed as the below equation (1), wherein Cpi-s is the coupling capacitance between the signal line 16 and the pixel electrode 21, Clc is a capacitance of LCs between the pixel electrode 21 and the counter electrode 11, and Cstr is an auxiliary capacitance between the pixel electrode 21 and the common electrode 24. EQU .alpha.=Cpi-s/(Cpi-s+Clc+Cstr) (1)
When the interlayer dielectric layer 25 is formed by employing the resin in this manner, the thicker layer can be obtained more easily than in the case of employing a chemical vapor deposition (CVD) procedure. Accordingly, the interlayer dielectric film 25 is formed as thick as possible to reduce the coupling capacitance while disposing the signal line 16 overlapping with the pixel electrode 21. Thereby, the area of the black matrix 18 which conceals a region not related with the display is reduced to increase a rate of the effective display area to the pixel area (aperture rate).
Since a position adjustment accuracy of an optical aligner for fabrication a current LCD is about 0.5 .mu.m, the amount of the overlapping between the signal line 16 and the pixel electrode 21 is required to be about 1 .mu.m.
When acryl-base resin having a relative dielectric constant of 3 is used as the interlayer dielectric film 25, the required film thickness thereof is 3 .mu.m or more for obtaining as small a crosstalk as that of an LCD in which silicon nitride is used as the interlayer dielectric film 25 without overlapping. The layer thickness of about 4.5 .mu.m is necessary in view of the case wherein the patterned dimensions of the pixel electrode 21 and the signal line 16 are larger than expected.
In JP-A-10(1998)-206888, a material having a negative-photosensitivity group is generally used as the photosensitive color resin. Accordingly, a portion exposed by using the photomask 100 as shown in FIGS. 3A and 3B is hardened to form a hardened portion 40. However, the color resin does not allow the light for exposure to reach to a sufficiently deep portion, and only the surface of the color filter 19 is hardened.
The development under such a situation causes the color filter 19 to have an overhang shown in FIG. 3C because the deeper portion is not sufficiently hardened to be easily etched.
If the diameter of the through-hole of the overcoat 20 is larger than that of the color filter 19 when the overcoat 20 is applied and pre-baked on the color filter 19 for the exposure, the development and the baking, the through-hole has such an overhang at the central portion as shown in FIG. 3D to worsen the contact between the pixel electrode 21 and the drain electrode 16b.
In order to prevent this occurrence, the diameter of the through-hole of the overcoat 20 is required to be smaller than that of the color filter 19 as shown in FIG. 3E. An overlapping margin for exposure must be considered to enlarge the diameter. Accordingly, such a problem is involved that the area of the opening becomes narrower and the rate of the aperture rate becomes lower.
On the other hand, in JP-A-9(1997)-152625, positive photosensitive resin is generally used for forming the interlayer dielectric layer having the through-hole. This is because the removal of a non-exposed portion after the development, which may be generated by, for example, dusts mixed therein when the negative resin is used, can be prevented by employing the positive resin. If the removal of the non-exposed portion occurs, through-holes are formed in unintended portions to cause a short-circuit failure.
When, however, the positive resin is used, a portion not removed by development remains or an unreacted photosensitive group remains in the non-exposed portion to reduce a transmission factor. Even if light is irradiated on the entire surface to react the photosensitive group after the development, the transmission factor of the ordinary positive transparent resin is lower than that of the negative resin.
When, for example, positive acryl resin PC403 available from JSR Kabushiki Kaisha having a thickness of 4.5 .mu.m is used, a transmission factor in a visible light band is 95% in average. The transmission factor at the wavelength band between 350 and 420 nm in which the photosensitive group is reacted with light is especially lower, and a part of white problematically becomes yellow.